A conventional Internet Protocol Multimedia Subsystem (IMS) network is an architectural framework for delivering Internet Protocol (IP) services. The conventional IMS network may implement a signaling protocol to establish, modify, and/or terminate various forms of communication. One type of signaling protocol may be Session Initiation Protocol (SIP).
In a conventional IMS/SIP network, a set of user equipments (UEs) may use SIP and have a shared phone number. To distinguish each of the set of conventional UEs with the shared phone number, the set of conventional UEs may also undertake a registration procedure. The registration procedure may allow each UE to be assigned a unique contact address. Network elements in the conventional IMS/SIP network may use the unique contact address to send an INVITE request to each conventional UE when there is an incoming call to the shared phone number, such that the set of conventional UEs ring simultaneously. This allows a subscriber to answer the incoming call at any of the conventional UEs.
Nevertheless, a shortcoming of the conventional IMS/SIP network is that once the incoming call has been answered by a first person at a first conventional UE, a second person cannot simply pick up a second conventional UE to join the call, despite the fact that both conventional UEs have a shared phone number. Instead, the second person must dial a special access code from the second conventional UE to join the call. This deficiency stands in contrast to traditional Public Switched Telephone Network (PSTN) phone service, where the second person can pick up a handset at any other phone with the shared phone number to immediately join the call. Such a difference in operation by telephony devices in the conventional IMS/SIP network may be viewed as a critical deficiency by telecommunications providers and end users that expect feature transparency when migrating from PSTN to VoIP.